The goal of this research project is to understand the function of the gene mutated in Rieger syndrome I, the pitx2 homeobox gene, during odontogenesis. Human patients with Rieger syndrome I, a haploinsufficient disorder, have tooth anomalies, anterior segment ocular defects, and umbilical defects as cardinal features. Using gene targeting in embryonic stem cells, we generated a mouse model of Rieger syndrome I. Pitx2 null mice had severe abnormalities in craniofacial and tooth morphogenesis. Moreover, we found that pitx2 null mutant oral ectoderm failed to express fgf8 and had expanded expression of Bmp4, suggesting that pitx2 has a role in regulating these signaling pathways. We have generated two new isoform-specific pitx2 mutant alleles. Analysis of pitx2 allelic combinations revealed a differential sensitivity of the fgf8 and Bmp-mediated signaling pathways to pitx2 dosage. The first aim of this research program proposes to investigate the cellular mechanisms of pitx2 function in oral ectoderm using chimera analysis. We will also perform functional studies to probe the relationship between pitx2 and the Bmp4- and fgf8-signaling pathways. Finally, we propose to investigate pitx2c isoform-specific function in oral and dental epithelium. These studies will lead to fundamental advances in the understanding of the molecular genetic mechanisms underlying a human syndrome, Rieger syndrome I.